Oxidation of mercaptans by sulphur is an important access route to organic disulphides. This direct sulphuration with elimination of hydrogen sulphide in accordance with the reaction: 2 RSH+S.fwdarw.RSSR+H.sub.2 S is generally carried out in the liquid phase and catalyzed by organic or inorganic basic agents, in particular alkaline bases or amines. When it is carried out with solid sulphur in the presence of an amine, this reaction is essentially carried out discontinuously and is accompanied by the formation of higher polysulphides. See U.S. Pat. No. 2,237,625.
To avoid the use of solid sulphur, it has been proposed in French Patent No. 1,358,398 to bring the mercaptan directly into contact with a solution of sulphur in a solvent. The latter is preferably an organic disulphide and principally the disulphide which it is sought to produce. In this process, in which the solution of sulphur in the disulphide is prepared in advance in a special vat, the amine serves not only to catalyze the oxidation reaction but also to increase the solubility of the sulphur in the disulphide. This obtains solutions which are very concentrated in sulphur.
A great improvement in this synthetic route to organic disulphides was the subject of French Patent No. 2,130,985. It comprises adding to a stirred reactor, sulphur in the liquid state above the liquid reaction phase containing the basic catalyst in solution, for example an amine such as triethylamine. This mode of operation enables organic disulphides to be obtained which are devoid of any other polysulphide. It is possible to carry out a simplified continuous process.
Organic polysulphides RS.sub.n R (n&gt;2), particularly aliphatic, cycloaliphatic or aryl polysulphides, and most particularly dialkyl polysulphides, are products of commercial value for diverse applications, particularly as extreme pressure additives in cutting oils.
Their preparation by reacting elementary sulphur with a mercaptan is known. This reaction: EQU 2 RSH+(n-1) S.fwdarw.RS.sub.n R+H.sub.2 S
requires the presence of a basic catalyst which can be an amine, an alkanolamine, an inorganic base, a mercaptide or an alcoholate, or else a catalyst formed by the combination of a mercaptan with an alkylene oxide and an alkaline base. Processes of this kind have been disclosed for example in U.S. Pat. Nos. 2,237,625; 2,237,627; 3,022,351 and 3,392,201 and in French Patent No. 1,381,265; British Patent No. 1,162,334; German Patent No. 2,938,156 and French Patent No. 2,607,496.
The use of magnesium oxide as catalyst has recently been claimed in U.S. Pat. No. 4,564,709. Being solid and insoluble in the liquid reaction media obtained by reacting elementary sulphur with mercaptans or with organic polysulphides, a catalyst of this kind has the advantage of not contaminating the polysulphides produced in these reactions. However, magnesium oxide used in powder form requires very effective filtration of the reaction products, which makes the use of a process of this kind for continuous production of organic polysulphides very sensitive.
The preparation of polysulphides RS.sub.n R can also be carried out by the action of elementary sulphur on organic polysulphides RS.sub.n 'R having a lower sulphur-rank (2.ltoreq.n'&lt;n), for example di-, tri- or tetrasulphides to produce higher polysulphides. This possibility is indicated for example in French Patent No. 1,381,265, amines being present as catalysts.
The preceding references are hereby incorporated by reference.